The present invention relates to valves in general, and more particularly to a butterfly valve housing and to a method of manufacturing the same.
There are already known various constructions of butterfly valves and of their housings. So, for instance, it is known so to construct the butterfly valve that its housing is insertable between the flanges of a pipeline, being clamped or otherwise held in position between such flanges. The valve housing is provided with transverse openings wnich are centered on a transverse axis that extends substantially normal to the axis of the pipeline, and which rotatably support a shaft or trunnions supporting a substantially disc-shaped butterfly valve member, so that the latter can be turned through substantially 90.degree. about the transverse axis between its open and closed positions. The valve housing bounds a substantially cylindrical internal opening that is centered on an axis that substantially coincides with the axis of the pipeline, and the disc-shaped butterfly valve member is constructed to fit into this internal opening, there being provided sealing means either on the disc-shaped butterfly valve member, or in the internal opening of the valve housing, or both, to seal the gap between the disc-shaped butterfly valve member and the internal surface of the valve housing at least in the closed position of the disc-shaped butterfly valve member.
In this connection, it has already been proposed, for instance, in the U.S. Pat. No. 3,958,595, to encapsulate the exposed surfaces of the valve housing in a corrosion-resistant polymeric plastic coating layer, this coating layer extending into the transverse openings as well. This coating layer is relatively thin and thus has to be bonded to the underlying surfaces to maintain its shape in conformity with the shape of the underlying surfaces of the valve housing. Experience has shown, however, that during repeated thermal expansion and contraction cycling of the valve housing as it often occurs during the operation of the butterfly valve, because of the usually different coefficients of thermal expansion of the material of the valve housing and that of the coating layer, the aforementioned coating layer may become detached from the underlying surfaces or may crack or even peel off, due to its relative thinness and reliance on the underlying valve housing for support, and aggressive agents may then penetrate to the valve housing and cause corrosion of the latter. Moreover, since the coating layer is so thin, it will track even minute surface irregularities of the underlying portions of the valve housing, so that those regions of the surface of the metallic valve housing where the coating layer is required to be devoid of such irregularities must be machined or otherwise shaped prior to the application of the coating layer to the metallic valve housing, or the coating layer must be machined, thermally sized, or otherwise treated to remove such irregularities from the exposed surface of the coating layer at such regions. In each instance, there is required one or more machining or treating operations to achieve the desired surface quality of the coating layer at the affected region, which makes the manufacture of the valve housing rather time-consuming, cumbersome and expensive.
It is also known, for instance, from the U.S. Pat. Nos. 3,537,683, 3,738,383 and 3,990,675, to completely embed reinforcing rings or other core bodies in outer bodies of an elastomeric sealing material which is molded around such core bodies and may be but not necessarily is bonded thereto, and which does not require, and hence usually does not have, a high-quality surface finish. Because of the elasticity of such materials, the aforementioned problems arising from the differential expansion are not encountered here, but the body of the elastomeric material is incapable of forming a self-supporting shell around the core body. Furthermore, so far this approach has been perceived to be usable only with such elastomeric sealing materials, particularly in view of the fact that the tremendous pressures occurring during injection molding of other types of synthetic plastic materials could break, distort or otherwise damage the core body. Such core bodies are usually supported by pins or similar structures in the respective molds during the molding operation; however, such structures completely obstruct the regions which they engage (typically the valve disc shaft or trunnion openings), not enabling the molded material to reach such regions, so that such regions will be exposed to the environmental influences after the supporting structures have been removed from the molded body following the molding operation. This is particularly deleterious when the above-mentioned transverse openings are used for receiving the supporting pins during the molding operation, as they very often are, since the surfaces bounding such transverse openings are then vulnerable to attack during the use of the valve since they are left uncovered by the molded body.
Finally, it is known, for instance, from the U.S. Pat. Nos. 3,318,567 and 3,904,173, to provide linings, usually of an elastomeric materials, on and adjacent the internal surface of the valve housing which surrounds the internal opening of such valve housing that communicates with the consecutive sections of the pipeline and receives the disc-shaped butterfly valve member. Such linings also usually do not have a high-quality surface finish since they do not require the same due to their elasticity and their attendant capability of conforming in shape to the surfaces with which they come into contact. Moreover, these known linings are incomplete, that is, they do not encase the portions of the valve housing which face the exterior of the valve or even completely those portions which face the flanges of the consecutive pipeline sections. Thus, such exposed portions of the valve housing will remain vulnerable to attacks by environmental influences. Furthermore, problems will be encountered with maintaining such linings in their desired positions. Last but not least, it would be impossible to use this approach in connection with so-called engineering grade synthetic plastic materials, since the very high pressures encountered during the molding of such inner linings would damage the underlying metallic elements.